This invention relates generally to methods for exposing selected small regions of layers of materials on needle shaped objects, and more particularly to fabrication of thermocouples and probes for scanning force microscopes.
As is known in the art, the manufacture of probe tips for what are known as scanning force microscopes requires precise control of dimensions and shapes. The probe tips, known as microprobes, are generally elongated metallic structures having highly sharpened tips containing the microprobe detector. Examples of various microprobe detectors known in the art include tunneling current, magnetic field detectors and thermal detectors. In particular, typical thermal microprobes use sharp needles comprised of two different metals, known as a bimetallic thermocouple, where the area of the contact region between the two different metals is one of the important control features, along with the atomic composition of the two metals and the amount of contamination in the contact region. It is difficult to control the contact area of the bimetallic thermocouple because of the difficult geometry of the sharp needle tip.
A known method for making bimetallic thermocouples for use in scanning microprobes (known as thermal microprobes) is to coat a sharpened needle with a first dielectric layer, a first metal layer and a second dielectric layer. The tip of the needle is held close to a metal plate and a large enough voltage is applied to the first metal layer to cause the second dielectric layer to rupture, causing a contact region to form. The second metal layer of the thermocouple is deposited to form the required bimetallic layer.
However, such a method is not effective for providing reproducible thermal microprobes because the area of the contact region between the two different metals is neither carefully controlled, adjustable nor reproducible. Thus the manufacturing yields of good thermocouple probes is reduced, consequently the cost of thermal microprobes is increased, and the sensitivity of the thermal microprobes varies from probe to probe, requiring difficult calibration procedures.
Thus, the prior art has two distinct problems with bimetallic layers for thermal microprobes. The first problem is forming small reproducible low resistance contact regions on sharp needle like structures, which may result in poor manufacturing yields due to variable contact resistance between the two metal layers, and incompletely opened contacts. The second problem is that the contacts are not of a preselectable or uniform size, resulting in various sized contact regions and variable thermocouple performance and sensitivity. The smaller the contact area between the two metal layers the less the thermal mass of the thermocouple and the faster the temperature response can be. This feature is particularly important in the scanning force microscope field since it impacts the maximum possible scanning speed. Such small reproducible thermal microprobes may be used in scanning force microscopes to perform real time measurements on operating integrated circuits, in effect providing the ability to watch a microscopic integrated circuit function in a normal fashion.